In a technique of the sound-source localization system using a beam-forming theory, a microphone array is constructed by using a plurality of microphones, and thereby a sound only in a target direction is extracted by realizing a sharp directivity by way of operations and synthesis from amplitudes and phases inputted into the respective microphones. For example, Japanese Patent Application (translation of PCT international application) Publication No. 2005-521283 or Japanese Patent Application Publication No. 2011-15050 proposes a microphone array which is used for such a sound-source localization system.
In the microphone array disclosed in Japanese Patent Application (translation of PCT international application) Publication No. 2005-521283 or Japanese Patent Application Publication No. 2011-15050, a plurality microphones are arranged on a plurality of radial lines with predetermined pitches, and as a result, the microphones are respectively located on a plurality of concentric circles. However, the microphone array is designed not to orient each of the plurality of radial lines to a center of the concentric circles. Thus, a circular microphone array having a pseudorandom microphone distribution which is considered to be favorable in the field of acoustic beam-forming technique can be obtained.
If a phase difference cannot be sufficiently secured when the sound-source localization system controls phases, the directivity of beam for sound source localization is reduced so that an accuracy of the sound source localization is lowered. This occurs in the case that a size of the array or a distance between microphones is small relative to a wavelength corresponding to target frequency of the sound source localization. On the other hand, if trying to sufficiently secure the phase difference by widening the distance between microphones, there is a risk that the phase is shifted by more than 360 degrees to cause an excessively large phase difference in a high-frequency region, so that a side-lobe level is increased.
In other words, if the microphone array as typified by Japanese Patent Application (translation of PCT international application) Publication No. 2005-521283 and Japanese Patent Application Publication No. 2011-15050 is applied to a sound localization system for an electric power-train unit of an automotive vehicle or the like, the vehicle generates noises in a wide frequency range approximately from several hertz (Hz) to several kilohertz (kHz), and hence, the microphone array having such one size cannot deal with a sound localization for these noises having the wide frequency range.